The proposed project is focused on the hormonal regulation of testicular function in the rat. Against this background, the broad objective of the proposed study is to establish the physiological significance of periodic fluctuations in the blood level of two hormones known to control testicular function--luteinizing hormone (LH) and testosterone. An initial set of experiments is planned to define the effects of external cues on moment-to-moment changes in the blood level of these hormones in the intact or castrated rat. The goal inherent in this study is to establish the magnitude and time domain of "secretory" episodes in LH and testosterone and to assess the relative importance of external vs internal signals in evoking episodic fluctuations in these hormones. The second series of experiments will examine the negative feedback action of testosterone (and other testicular steroids) on LH secretion in the intact and castrated male rat. These studies will assess amplitude and frequency variations in LH release during continuous or intermittent intravascular infusion of testicular steroids in the absence and presence of the testis. The third set of studies is designed to match testicular steroid production and interstitial cell LH binding with dynamic changes in LH administration. The aim of this work is to assess the response of the in vitro perfused testis to amplitude and frequency changes in LH signals in terms of LH-receptors and the production of certain C18, C19 and C21 testicular steroids. Hence, the second and third experiments will emphasize frequency (i.e., pulse duration) and amplitude (i.e., pulse strength) modulations in the chief hormones regulating the pituitary-testicular axis. In the fourth objective, I will exploit the use of continuous or intermittent intravascular infusion of testicular and/or hypophyseal hormones to study the reinitiation of spermatogenesis in the long-term hypophysectomized rat. Taken together, the proposed studies are designed to characterize the quality and quantity of the physiological signals that regulate LH release and testicular steroid production under circumstances designed to inhibit or maintain spermatogenesis in the rat.